Probing the Cosmic Dawn

The first black holes and stars

Our understanding of cosmology has expanded greatly in recent years. On the one hand, detailed observations of the cosmic microwave background have shown us a ‘baby picture’ of the universe as it was only 300 000 years after the Big Bang. During the next roughly half billion years, structures on all scales began to collapse under gravity and the first galaxies formed.

How will the SKA peer in to this largely unknown era?

Space and ground-based telescopes have given us unprecedented views of distant Universe. Optical and infrared telescopes have imaged galaxies at distances over 13 billion light years from Earth, at a time when the Universe was less than a billion years old. The promise of large infrared telescopes in space and giant optical telescopes on the ground should reveal even more distant and sensitive views of early galaxies.

Space telescopes like Planck that observe the Cosmic Microwave Background radiation have mapped the light from the very early Universe, just after the moment of the Big Bang.

The Stunning Planck all sky survey

One of the last frontiers in cosmology is to explore this cosmological dawn when the first galaxies formed, and the SKA is the most sensitive radio telescope that will conduct such studies,

This period in the early Universe that will be studied started around 380,000 years after the Big Bang, when the Universe was mostly dark until the first galaxies began to shine.

It is an age in which these proto-galaxies and quasars formed and has been one of the most difficult epoch’s of the Universe to explore, as these objects are exceptionally faint and much of their light is absorbed by intervening matter as it travels toward us.

The sensitive optical and infrared sky surveys have shown that these young galaxies are unlike anything we observe in the local Universe, with stars that could be orders of magnitude larger than our own Sun, and with much shorter lifetimes.

The SKA, through imaging of the atomic Hydrogen gas, will provide pictures of the period during and after the formation of the earliest sources of light in our Universe, providing the first detailed measurements ever of the conditions under which they formed occurring in this mysterious time.

Snapshot simulations of the HI universe evolving with time. The dark regions correspond to highly ionised regions (such as those around protogalaxies) and the bright regions are dense, neutral pockets of gas. Credit: Steve Furlanetto et al 2003, MNRAS

So what is the epoch of reionisation?

Prior to the first structures such as galaxies and stars forming in the Universe most of the gas, which was predominantly Hydrogen in the early universe, was fairly evenly distributed and electrically neutral. This is known as the Dark Ages.

A special property of neutral hydrogen is that it produces weak radiation at a wavelength of 21 cm, which in theory can be used to study this period, but in practice, its signal is very faint, and difficult to detect through the Earth’s ionosphere. However, once the first celestial objects, proto-galaxies and stars, started to form through gravitational instabilities, their light ionised pockets of the gas around them, switching off the 21 cm emission from these regions. Astronomers have labelled this important event as “reionisation”.

This era in the formation of the Universe has proven difficult to study. Not only are proto-galaxies extremely distant and faint, the key problem is that much of their light in optical and even out to the infra-red is absorbed as it travels toward us.

Tantalising clues from WMAP and the SDSS survey telescopes have scientists excited, as these objects display characteristics which are unlike anything they have seen before.

For example, the first proto-galaxies may form through different mechanisms than our own Milky Way, and the stars inside these objects may be hundreds of times more massive than our own sun.

What is the Epoch of Reionisation?

So what will the SKA provide?

The Square Kilometre Array will study the detailed properties of the first luminous objects in the universe, and be able to take snapshots of the 21 cm emission at many different epochs, before, during and after reionisation, yielding detailed information about the formation of the first structures in the universe. It will provide the best measurements available of the characteristics of the first light sources in the universe.